This invention relates to the art of removing material from metallic objects, such as in cavity sinking, boring, planing, deburring, polishing, and the like, and has particular relationship to such material removal by electrochemical machining. It is an object of the invention to provide a facility for removing material from complex metal structures at a relatively low cost.
A typical situation to which the invention is applicable is in the fabrication of cast large pump rotors with multiple helically-shaped passages. Such rotors are used to circulate liquid sodium. It was necessary to release trapped sand particles and to eliminate sharp edges associated with small cavities. Manual surface processing is costly and, in any event, only reaches over about one-half the surface which requires processing.
Typical of another structure to which this invention is applicable is the tube-support plate of the steam generator of a nuclear-reactor power supply. Typically, such a plate is composed of carbon steel, is about 11 feet in diameter and has 9300, three-quarter-inch holes and 9300, one-half-inch holes, 18,600 holes in all. These holes are drilled through the plate. The purpose of these holes is to support the hairpin heat-exchanger tubes composed of INCONEL alloy. It is necessary that the burrs on the edges of these holes be removed to preclude scratching the tubes and reducing their lives. To preclude damage to the tubes, it is also desirable that the edges of the holes be rounded off.
The prior art practice is to deburr the 18,600 holes manually with power tools. This operation consumes about forty-eight man hours of labor. Shot-blasting machines have proven unsatisfactory.
It has been proposed that electrochemical machining be applied for the deburring operation to remove unwanted material as in reverse plating. In accordance with the teachings of the prior art, electrochemical machining is carried out by a fixed electrode. This requires precision electrodes which must be maintained precisely spaced from the work as the machining progresses. Typically, in fixed-electrode deburring the fixed electrode is positioned about 0.020 inches from the burr, an electrolyte is conducted at a high velocity between the burr and the electrode, and a high-current-density, low voltage, direct current is conducted between the electrode and the work. The burr material is deplated and carried away in the electrolyte, typically as a hydroxide. A difficulty with fixed-electrode deburring is that as the burr material is removed, the width of the gap between the electrode and the work increases, increasing the gap resistance and decreasing the current density and the rate of removal of material. Experience with electrochemical fixed-electrode deburring has led to the conclusion that the following parameters must be controlled.
1. Position and proximity of the electrode to the burr.
2. Velocity, pressure and direction of the electrolyte.
3. Applied direct current voltage.
4. Cycle time.
Parameters 2, 3, and 4 can readily be controlled, but parameter 1 presents serious difficulties and costs. Indeed, it has been found that in many cases the cost of the precision electrodes required and their position control is prohibitive. Particularly high is this cost where large and complex geometry parts, such as the pump rotor and the tube support plate, are to be processed or where the exact position of the unwanted material cannot be readily predicted.
It is an object of this invention to overcome the above-described difficulties and disadvantages of the prior art and to provide relatively low-cost electrochemical machining apparatus for removing material from work.
It is also an object of this invention to provide such apparatus which shall not require precision electrodes. It is a further object of this invention to provide such apparatus in whose use the position of the electrode with respect to, and the spacing of the electrode from, the work shall be automatically maintained. An additional object of this invention is to provide a method of electrochemical machining in whose practice the above difficulties and disadvantages of the prior art shall be overcome. It is an object of the invention to provide electrochemical machining apparatus and a method for deburring a plate having holes therein in the use of which the edges of the holes shall not only be effectively deburred, but shall also be rounded.